1. Field of the Invention
The present invention relates to a process of forming a semiconductor device, in particular, relates to plasma process such as a plasma-enhanced chemical vapor deposition, a sputtering, a reactive ion etching (RIE) and so on.
A process of forming a semiconductor device often performs plasma processes, such as a reactive ion etching (RIE), a plasma-enhanced chemical vapor deposition (p-CVD), a sputtering, and so on. The plasma etching ionizes gasses such as carbon tetrafluoride (CF4), sulfur hexafluoride (SF6), and so on with a radio frequency (RF) signal to make plasma. Ions and/or radials contained in the plasma may etch an insulating film of silicon nitride (SiN), silicon oxide (SiO), and so on. Such a dry etching inevitably accumulates charges on a surface of an object to be etched, which is often called as the charge-up. A prior patent document laid open No. JP-H10-079392 has disclosed a technique to couple electrons and positive ions captured within a silicon (Si) wafer during the plasm etching.
The charge-up described above raises electric potential of the surface of the object, which forms a potential difference between the surface and an interior of the object and causes a substantial current from the surface to the interior. This current sometimes induces damage in a semiconductor layer or a device, which increases a leak current. Also, the plasma sometimes causes fatal damages in the semiconductor layer by arcing. In particular, when the substrate is made of nitride semiconductor material, such as gallium nitride (GaN), which inherently shows extremely high resistivity; the charges induced during the plasma process easily concentrate on a surface of the wafer, or a center of a surface of the wafer. Accordingly, the process of forming a semiconductor device primarily made of nitride semiconductor materials is necessary to consider the charge-up during the plasma process.